How to store the date when the state of an object was changed?

Hi, I need to save the datetime when someone changes the Invoice State and I also need to store the user that made that change. So I need that whenever someone changes the state field that two new fields ('state_change_date', 'state_change_user') to be updated because I just need to store the last change. I tryed to figure it out how to do this, but I just found one way that I don't like that it is to look in all code an see where the state is changed and inherit all of this functions and change my new fields... If someone knows a better way I would appreciate =) Thanks